Methodology of Natsal-COVID Wave 2: A large, quasi-representative, longitudinal survey measuring the impact of COVID-19 on sexual and reproductive health in Britain

Background The National Surveys of Sexual Attitudes and Lifestyles COVID study (Natsal-COVID) was designed to understand the impact of COVID-19 on Britain’s sexual and reproductive health (SRH). Natsal-COVID Wave 1 survey and qualitative follow-up interviews were conducted in 2020. The Wave 2 survey was designed to capture one-year prevalence estimates for key SRH outcomes and measure changes over the first year of the pandemic. We describe the Wave 2 survey methodology and assess the sample representativeness. Methods Natsal-COVID Wave 2 was conducted March-April 2021; approximately one year after the start of Britain’s first national lockdown. Data were collected using an online web-panel survey administered by Ipsos. The sample comprised a longitudinal sample of Wave 1 participants who had agreed to re-contact plus a sample of participants residing in Britain, aged 18-59, including a boost sample comprising people aged 18-29. Questions covered reproductive health, relationships, sexual behaviour and SRH service use. Quotas and weighting were used to achieve a quasi-representative sample of the British population. Comparisons were made with recent national probability surveys, Natsal-3 (2010-12) and Natsal-COVID Wave 1 to understand bias. Results A total of 6,658 individuals completed the survey. In terms of gender, age, ethnicity, and rurality, the weighted Natsal-COVID Wave 2 sample was like the general population. Participants were less likely to be married or to report being in good health than the general population. The longitudinal sample (n=2,098) were broadly like participants who only took part in Wave 1 but were older. Among the sexually active, longitudinal participants were less likely to report multiple sexual partners or a new sexual partner in the past year compared to those who only took part in Wave 1. Conclusions Natsal-COVID collected longitudinal, quasi-representative population data to enable evaluation of the population-level impact of COVID-19 on SRH and to inform policy.


Background
This research note describes the methodology used to conduct Wave 2 of the National Surveys of Sexual Attitudes and Lifestyles COVID study (Natsal-COVID). It follows our previous work describing the Wave 1 study methodology 1 .
At the time of Natsal-COVID Wave 2 data collection (March-April 2021), COVID-19 lockdown restrictions in Britain were being eased following a second national lockdown between January and March 2021. Some form of restrictions and physical distancing requirements had been in place throughout the preceding year 2 . Physical contact with anyone outside one's household or support bubble was not permitted for the duration of that year.
Natsal-COVID survey Wave 1 and qualitative follow-up interviews were conducted in 2020 1 to understand early changes in sexual and reproductive health (SRH) service use and need, sexual behaviours, and relationships during this time [3][4][5][6] . Wave 2 aimed to capture SRH behaviour and outcomes during the first year of the COVID-19 pandemic, including sexual behaviours, sexual function, relationship quality, intimate partner violence, reproductive health outcomes and SRH service use. It was designed to produce one-year prevalence estimates of key SRH outcomes and behaviours. The study also aimed to measure change over time, both between-person variation through repeat cross-sectional analyses and within-person variation through longitudinal analyses. This paper describes the methods used in Wave 2 of Natsal-COVID and assesses the representativeness of the data.

Sample design
Natsal-COVID Wave 1 was an online web-panel survey conducted in July-August 2020, which used quotas and weighting to obtain a quasi-representative sample of 6654 people aged 18-59 years old living in Britain. The Wave 2 sample was drawn first from those who participated in Wave 1 and agreed to re-contact (the longitudinal sample). No quotas were set for this group. To complete the Wave 2 sample, new participants were sampled from Ipsos's online panels. Sample quotas were set by gender, age, region, and social grade. The new sample included a boost of 500 people aged 18-29 years old, ensuring an overall sample of 2000 participants in this age-group. The complete Wave 2 sample was designed to ensure overall representativeness of the population aged 18-59 years old by age, gender, region, and social grade. Full details of sample size calculations for Natsal-COVID have been reported elsewhere 1 . The target longitudinal sample size at Wave 2 was 4,000.

Ethical approval
We obtained ethics approval for the study from University of Glasgow Medical, Veterinary and Life Sciences College Ethics Committee (reference 20019174) and London School of Hygiene and Tropical Medicine Research Ethics committee (reference 22565). Participants provided informed consent to participate via an online consent form before starting the survey.

The questionnaire
The Natsal-COVID Wave 2 questionnaire was adapted from the Wave 1 questionnaire, the development of which has been previously described 1 . The dataset for Wave 1 can be found in the UK Data Archive 7 . Wave 2 additionally included questions about pregnancy, contraception changes, HIV testing, chlamydia testing, abortion, relationship formation and dissolution, and IPV with a focus on the year since the start of the first lockdown (Box 1). Questions relating to experiences of the COVID-19 pandemic were included, mainly drawn from other major COVID-19 studies 8,9 . The full questionnaire is available on the study website under 'Natsal-COVID Survey Questionnaire -Wave 2'. Natsal surveys have always involved sensitive question topics, e.g., sexual behaviours, reproductive health, SRH service use. The introduction of IPV questions using remote collection methods in Natsal-COVID Wave 2 required careful consideration regarding potential risk to participants and data quality, while acknowledging the importance of collecting these data during the COVID-19 pandemic. A variety of measures were put in place to mitigate the risks. We minimised the number of IPV items, avoided very sensitive questions (e.g., physical force) and included an explicit 'prefer not to answer' option at every question in the module. Immediately before sensitive question modules (including IPV), reminders about the voluntary nature of each question and confidentiality were displayed. Appropriate signposting to support services was provided after each sensitive module and at the end of the survey.

Sample recruitment
Wave 2 survey data were collected from 27 March 2021 to 26 April 2021. Approximately 150,000 panellists, including those from Wave 1 who were willing to be re-contacted, were contacted via email. Of those emailed, 38,731 started the survey; 79% came from Ipsos's own panel, with 'top-up' from three other panel providers used by Ipsos. Of these participants, 11,708 were ineligible or did not provide consent, 17,230 were diverted from completing the survey because their quota was full, 2,376 abandoned the survey before completion, 490 failed quality checks, and 269 experienced a technical error. In total, 6,658 participants completed the survey and are included in the analysis. Of these, 2,098 were longitudinal participants and 4,556 were new. The recruitment process is shown in Figure 1.

Quota filling and weighting of survey data
To increase numbers of participants aged 18-29 years old, all other quotas (gender, region, and social grade) were relaxed toward the end of fieldwork. Cross-sectional and longitudinal weights were produced to achieve a quasi-representative sample of the British population by gender, age, region, social grade, ethnicity, and sexual identity. The cross-sectional weight for the full Wave 2 sample had a weighting efficiency of 83.8% (see Ipsos Wave 2 Technical Report).

Gender in Natsal-COVID
Natsal-COVID is inclusive in its approach to gender, as described for Wave 1 1 . A total of 67 Wave 2 participants were classified as 'trans' where their reported gender identity was different to their sex described at birth. This included 26 trans men, 19 trans women, and 22 people who identified in another way (Figure 2), giving an overall percentage of 0.9% in the weighted sample. We present data for men (including trans men) and women (including trans women) in our analysis. Individuals who identified 'in another way' were included in analyses where the denominator is all participants but were not included in denominators for men or women.

Representativeness of the Natsal-COVID sample
The Natsal-COVID Wave 2 sample was compared with Wave 1, and with the following probability sample surveys to assess representativeness ( Table 1 and Table 2): the 2019 Annual Population Survey (APS) 10 (gender, age, region, ethnicity, marital status, education, disability), the 2018 Health Survey for England (HSE) 11 (general health and rurality), the 2018 APS report 12 (sexual identity), and the 2010-12 Natsal-3 study 13 (sexual behaviours). More recent versions of HSE and APS have been published since we carried out analysis of Natsal-COVID Wave 1 representativeness. However, we decided to use consistent comparators in our assessment of the Natsal-COVID Wave 2 sample to facilitate comparisons with  As expected, due to quotas and weighting, the weighted Wave 2 sample was similar to Wave 1 and external datasets for gender, age, region, ethnicity, and sexual identity (Table 1). Like Wave 1, the unweighted Wave 2 sample over-represented non-heterosexual identifying participants (men, 11.9%; women, 10.8%). However, the weighted sample (men, 3.8%; women, 3.6%) was comparable to 2018 APS (men, 3.0%; women, 2.6%). Sexual identity was not available in the APS dataset, so we relied on reported tabulated data for a comparable population estimate. We have therefore compared individuals aged 18-59 years old in Natsal-COVID to those aged 16+ (i.e., no upper age limit) in APS. The over-representation of non-heterosexual identities in the unweighted Natsal-COVID sample can be partially explained by the younger age range of participants.
Regarding other sociodemographic variables, patterns were largely similar between Wave 1 and Wave 2. The Wave 2 sample under-represented participants who reported being married (40.4%) compared to the 2019 APS (47.5%) and under-represented those who reported 'very good' or 'good' general health (73.3%) compared to the 2018 HSE (79.9%).
The weighted proportion of Wave 2 participants reporting any previous partnered sexual experience (not necessarily involving genital contact) (93.9%) was lower than in Natsal-3 (98.8%). Among participants with at least one sexual partner in their lifetime, there was a higher proportion of Natsal-COVID Wave 2 participants reporting zero partners in the past five years (16.9%) compared with Natsal-3 (3.8%).
To characterise the sample of individuals who participated in both Waves of Natsal-COVID, we compared the unweighted and weighted characteristics and behaviours (reported at Wave 1) of those who did not participate in Wave 2 (n=4,556) with those who did (the longitudinal sample; n=2,098) ( Table 3). For most sociodemographic characteristics, the longitudinal sample was similar to those who participated only in Wave 1. However, participants in the youngest age group (18)(19)(20)(21)(22)(23)(24) were under-represented in the longitudinal sample (5.1% unweighted; 9.5% weighted) compared to the wave 1 only sample (19.8% unweighted; 17.2% weighted). Conversely, participants in the oldest age group (45-59) were over-represented in the longitudinal sample (49.5% unweighted; 35.4% weighted) compared to Wave 1 only participants (26.4% unweighted; 29.1% weighted). Among sexually active participants, a smaller proportion of the longitudinal sample reported multiple partners between July 2019 and July 2020 (10.9% unweighted; 11.3% weighted) compared with Wave 1 only participants (20.0% unweighted; 17.7% weighted). The longitudinal sample were also less likely to report any new partners in the same timeframe (19.2% unweighted; 23.8% weighted) than those who only took part in Wave 1 (30.8% unweighted; 28.9% weighted).

Discussion
Natsal-COVID is a large, multi-wave, national study that was undertaken when data were urgently needed to understand the impact of the pandemic on SRH services and inform policy. Initial findings from Natsal-COVID Wave 1 have been used in SRH policy and practice in Britain 14,15 . Natsal-COVID Wave 2 data generated one-year prevalence estimates for a range of key SRH behaviours and outcomes one year after the start of the first COVID-19 lockdown in Britain. The second wave of the survey also provides an opportunity to examine change over time during the first year of the pandemic.         Legend: CI=confidence intervals. Key sociodemographic characteristics and reported sexual behaviours were generally similar in the weighted Natsal-COVID data when compared to external probability surveys and Natsal-3. However, we noted bias in several important characteristics that remained after weighting. The Natsal-COVID Wave 2 sample under-represented individuals who were married and those who self-reported 'very good' or 'good' general health.
Among those with at least one sexual partner in their lifetime, a higher proportion of Natsal-COVID Wave 2 participants reported having no sexual partners in the past five years compared with Natsal-3 (conducted ten years ago). This difference could reflect pandemic restrictions, differences in the question wording, mode effects (i.e., online versus in-person), sampling bias or, most likely, a combination of these factors.
Prior to weighting, like Wave 1, Natsal-COVID Wave 2 had a higher proportion of participants identifying as nonheterosexual compared to 2019 APS. The over-representation of people with non-heterosexual identities in the unweighted Natsal-COVID data is consistent with previous web-panel surveys 16,17 . However, weighted percentages of non-heterosexual individuals were comparable between Natsal-COVID Wave 2 and 2019 APS.
The profile of participants who took part in both waves of Natsal-COVID (the longitudinal sample) demonstrated high survey attrition, particularly among young people. The sample bias is likely due to quotas not being applied to longitudinal sample recruitment and a lower response propensity among younger adults. Among sexually active participants, the longitudinal sample were less likely to report multiple sexual partners in the past year or a new sexual partner in the past year compared to participants who only participated in Wave 1 of Natsal-COVID. These differences in sexual behaviour estimates are likely attributable to sample bias predominately driven by age. Researchers conducting longitudinal analysis using Natsal-COVID data should bear in mind sample attrition and its associated bias when interpreting findings. The application of weights to the longitudinal sample reduced the magnitude of difference but did not correct them entirely.
Natsal-COVID did not use probability sampling methods and therefore inference to the general population should be undertaken with caution 18 . There are known sources of bias in web-panel surveys that may affect survey estimates 17,19 . Although the Natsal-COVID findings are likely to be largely generalisable, caution in the interpretation of prevalence estimates is advised particularly when analysing the longitudinal sample.
In conclusion, the Natsal-COVID Wave 2 survey has enabled us to identify impacts of the COVID-19 pandemic throughout the year following the first national lockdown in Britain and to monitor change over time in the same period.

Data availability
Underlying data The Natsal-COVID Wave 1 dataset has been deposited with the UK Data Archive (an open access repository) with safeguarded access (SN 8865 -National Survey of Sexual Attitudes and Lifestyles COVID-19 Study, 2020). The dataset is available to users registered with the UK Data Service. http://doi. org/10.5255/UKDA-SN-8865-1 7 .
Wave 2 data will be deposited at the same location within 10-12 weeks. Given the sensitive nature of the content of the data, a thorough disclosure risk assessment and the application of disclosure control measures are necessary prior to safe deposit. In the meantime, interested researchers or reviewers may contact the Natsal team (natsal@ucl.ac.uk) for interim access, with appropriate considerations about confidentiality and data protection.